1. Field of the Invention
The present invention relates to an aberration correcting apparatus for an information recording medium play-back apparatus. The information recording medium is generally an optical disc or similar medium.
2. Description of the Prior Art
In recent years the digital video (or versatile) disc (DVD) has attracted much attention as a large capacity digital information storage medium. Digital information such as animation, sound, computer information, or the like can be recorded on an optical disc having the same diameter, 12 cm, as a compact disk (CD) but with a recording density 6-8 times larger than the CD.
The DVD employs a smaller track pitch than the CD. The smaller track pitch increases the recording density and when coupled with a reduction in pit size a high density recording of approximately 5 G-bytes per side of the DVD has been achieved.
To read the high density recorded information of the DVD, a spot diameter of a laser beam employed to read the optical disk, is reduced to a size less than the laser beam spot diameter used for the CD. To produce a laser beam spot with a smaller diameter for the DVD, the wavelength of the laser beam source is set to 650 nm, which is shorter than the 780 nm laser beam wavelength used for play-back of the CD. The numerical aperture of an objective lens of the DVD play-back apparatus is set to 0.6, which is larger than the 0.45 aperture used in the play-back of the CD.
Central to the operation of the high-density recording disc is the concept of a tilt-angle. A tilt-angle is defined as the angle of deviation from a perpendicular surface of the optical disc with respect to the optical axis of the optical pickup. The tilt-angle causes a wave front aberration, mainly coma aberration, resulting in a deterioration of the S/N ratio of the reproduce signal due to the aberration (tilt-angle). The tilt-angle may result from one of several sources, such as warping of the surface of the optical disc or a tilting of the optical apparatus. As a result of the increase in the numerical aperture, the amount of the tilt-angle allowed is decreased.
The laser beam spot diameter is proportional to the wavelength of the laser beam source and inversely proportional to the numerical aperture of the objective lens. Therefore, by increasing the numerical aperture without changing the wavelength of the laser beam source, the recording density can be increased. As a result of increasing the numerical aperture, the angle that the surface of the optical disc is allowed to deviate from the perpendicular, with respect to the optical axis of an optical pickup, is reduced.
The optical discs, such as the DVD and CD, are slightly warped due to their own weight or to manufacturing conditions. However, if an optical disc has a large warped surface its tilt-angle may be large and exceed a tolerable level so that the S/N ratio deteriorates and, therefore, an excellent reproduction signal cannot be obtained.